Is there is reliable way to find out which revision of ECMA-262 my browser supports?

How开发者_StackOverflow社区 do I find out if my browser supports Javascript 1.5 or not? using javascript of course. (or should I have googled or binged some more?)

For official “JavaScript”, that is Mozilla's implementation of ECMA-262, you can use the type attribute to detect version:

<script type="text/javascript;version=1.5"> v= '1.5'; </script>
<script type="text/javascript;version=1.6"> v= '1.6'; </script>
<script type="text/javascript;version=1.7"> v= '1.7'; </script>
   ...

This also works in Opera, (recent versions of which claim to support JavaScript 1.5). IE and Webkit will not execute any script with a version parameter in its type.

This is similar to the old-school language attribute:

<script language="javascript1.5"> v= '1.5'; </script>
   ...

This is generally considered obsolete, but it does work in more browsers. Recent Webkit browsers claim to support 1.7 and all IEs claim to support 1.3 only.

IE has its own versioning scheme for JScript, which you can sniff for through conditional comments:

<script type="text/javascript">
    /*@cc_on @if (@_jscript_version>=5.5)
        v= '5.5';
    @end @*/
</script>

Other browsers you can't find out. As for ECMA-262 version, all modern browsers support the baseline of Third Edition (yeah, OK, there are small differences and outright bugs, especially in IE, but still). No browser supports large amounts of Fifth Edition yet, though that will come. (There was no Fourth Edition.)

None of this is very useful. As you can see, version numbers are all pretty woolly and don't really reflect what the browser supports terribly well. You are generally better off doing capability-sniffing. For example if you want ECMA-262-5's JSON features, see if window.JSON exists. If you want function binding, see if .bind exists on your function. You can often detect the lack of such features and patch in a native-JavaScript fallback for when they aren't available.

The syntactical features of JavaScript, however, can't be sniffed for. Just including something like a getter/setter definition in a block of JavaScript code will immediately give a syntax error in IE before you could sniff for their availability. But then this is usually not too big a problem because in practice you can't use these newer features anyway.

Its typically not worth trying.

If you want to make use of a specific feature, query to see if that feature is supported instead.

Even then, there's no guarantee that the feature is implemented correctly! ;-)

Case in point... IE supports document.getElementById(id); however unless you are using IE8 in IE8 Standards mode it can return one of many wrong results.

Another way you can do it, you can put inside try block some of it's syntax. for example:

try{
  "use strict";
  let x=1;//if it's works, then you have some supprot
  console.info("Good news, ecma6 works ");
}catch(e){
  console.warn("Sorry, but ecma6 still doesn't supported");
}

You can specify which version of Javascript should evaluate a block of code like this:

 <script language="javascript1.5">
 //this code gets run
 </script>

More info here.

There are scripts out there that will generate a float containing the Javascript version. Here's a nice read on detection with Javascript: https://developer.mozilla.org/En/Browser_Detection_and_Cross_Browser_Support

Here is such script: http://www.docsteve.com/DocSteve/Samples/JS/js_version.html